The present invention provides an improvement in pavement marking methods of the type taught in Harrington, U.S. Pat. No. 3,410,185; Harrington and Jorgensen, U.S. Pat. No. 3,664,242; and Condon and Harrington, U.S. Pat. No. 3,914,468. A basic part of these methods, as described in the first two listed patents, is the application of solid particulate thermosoftening marking materials to paved surfaces in the presence of heat to form continuous markings. Such methods made possible for the first time the formation of markings on paved surfaces that are ready to bear traffic within seconds after application.
Condon and Harrington obtained longer-lasting pavement markings by forming a bonding layer on the paved surface prior to application of the particulate thermo-softening marking material. The bonding layer was formed by coating onto the paved surface a thin bonding layer of low-viscosity film-forming bonding material which readily wets the paved surface, then advances to a non-spreading form, and ultimately advances to a tough film that adheres well to the paved surface.
The present invention provides a further increase in the life of pavement markings formed by the general procedure described. The invention is directed particularly to methods of pavement marking in which the marking material comprises solid particles that include a polyamide condensation product of dimerized fatty acid and polyamine. Briefly, the improvement comprises forming a bonding layer prior to application of the solid particles of marking material, the bonding layer being formed from a low-viscosity liquid coating composition that comprises 100 parts of a thermoplastic polyamide condensation product of a dimerized fatty acid and an amine, and at least 50 parts of volatile solvent in which the condensation product is dissolved; the solvent evaporating from the liquid coating composition at a rate of at least 0.20 gram/square centimeter/hour at a temperature of 24.degree. C and a relative humidity of 50 percent. Preferably, the liquid coating composition also includes an adhesion-promoting organosilane, the organic moiety of which is preferably potentially reactive with reactive functional groups in the polyamide of the solid particulate marking material. Also, for best results, the polyamide of the liquid coating composition is a low-molecular-weight material, as indicated by a low melt-viscosity, such as less than about 1000 centipoises at 160.degree. C.
Best results in the invention are also obtained when the polyamide of the solid particulate marking material, i.e., the topcoat material applied over the bonding layer, has a high molecular weight, as indicated by a high melt-viscosity, preferably 2000 centipoises or more, at 190.degree. C. Such high-molecular-weight polymers are more tough and flexible, though their flow during application is inhibited by their high molecular weight. However, with bonding layers applied according to the present invention, marking materials containing such high-molecular-weight polyamides have been applied to obtain highly durable markings.
While the full explanation for the improved durability obtained by the invention is not known, it is theorized that the polyamide of the bonding layer and the polyamide of the particulate marking or topcoat material coated over the bonding layer become intermixed during the marking operation, and that this intermixing is a compatible one that promotes good bonding and long useful life. Further, the proposed reaction of the organic moiety with reactive functional groups, such as amine groups, carried on the polyamide further improves the bond between the bonding layer and top layer. In addition, the silane moiety of the adhesion-promoting agent is believed to provide increased adhesion to the substrate, particularly concrete substrates, which include hydroxyl groups. Such adhesion between the silane moiety and pavement may be improved by hydrolyzing of the silane to silanol groups, which can occur in the presence of moisture.
Whatever the explanation, significant increases in durability have been obtained. For example, some side-by-side tests of markings formed using bonding layers as taught in the Condon and Harrington patents with markings formed according to the present invention have shown two-fold and greater improvements in durability. Such improvements significantly increase the scope of utility for marking methods of the general type which use particulate thermosoftening marking materials applied through a flame.